This invention is in the field of soil and groundwater treatment. This invention relates generally to processes and devices for reducing or eliminating perfluoroalkyl compound (PFC) concentrations in soil and groundwater.
Perfluoralkyl compounds such as PFOS (perfluoralkyl sulfonate) and PFOA (perfluoralkyl octanoic acid) are human-made substances, not naturally found in the environment, which do not hydrolyze, photolyze, or biodegrade in groundwater or soil. The compounds have been used as surface-active agents in a variety of products such as fire-fighting foams, coating additives and cleaning products. The toxicity and bioaccumulation potential of PFOS and PFOA, however, indicate a cause for concern. Studies have shown they have the potential to bioaccumulate and biomagnify up fish food chains. The products are readily absorbed after oral intake and accumulate primarily in the serum, kidney, and liver. Health-based advisories or screening levels for PFOS and PFOA have been developed by both the EPA and by an increasing number of States (Alaska, Maine, etc.) and European Countries (Finland, Sweden, Netherlands). Within the USA, Canada, and Europe (EU) there are an estimated 1000 sites with soil contamination (soils and groundwater) which have been used for fire foam training for aviation crashes.
Perfluoronated alkyl compounds are exceptionally stable because of the fully fluorinated bonding to the carbon atoms. They incorporate a long 8-carbon chain that is both lipid- and water-repellent. With a volatility at over 500° C., melting point at greater than 400° C., and boiling point that is not measurable, PFOS is used as surface-active agents in various high-temperature applications. The 3M Company, the primary manufacturer of PFOS, completed a voluntary phase-out of PFOS production in 2002 (ATSDR 2009; UNEP 2007).
Physical and chemical properties of PFOS and PFOA are provided in Table 1 (ATSDR 2009; Brooke et al. 2004; Cheng et al 2008; EFSA 2008; EPA 2002; UNEP 2006):
TABLE 1PropertyPFOS (Potassium Salt)PFOACAS Number2795-39-3335-67-1Physical Description (physical state at roomWhite PowderWhite powder/waxy whitetemperature and atmospheric pressure)solidMolecular weight (g/mol)538 (potassium salt)414Water Solubility (mg/L at 25° C.)570 (purified), 3709.5 × 103 (purified)(freshwater), 25 (filteredseawater)Melting Point (° C.)>40045 to 50Boiling Point (° C.)Not measurable188Vapor Pressure at 20° C. (mm Hg)2.48 × 10−60.017Air water partition coefficient (Pa.m3/mol)<2 × 10−6Not availableOctanol-water partition coefficient (log Kow)Not measurableNot measurableOrganic-carbon partition coefficient (log Koc)2.572.06Henry's law constant (atm m3/mol)3.05 × 10−9Not measurableHalf-LifeAtmospheric: 114 daysAtmospheric: 90 daysWater: >41 years (at 25° C.) Water: >92 years (at 25° C.)Photolytic: >3.7 yearsPhotolytic: >349 daysAdditional properties and molecular structures are provided in Table 2:
TABLE 2PropertyPFOSPFOAMolecular FormulaC8HF17O3SC8HF15O2 Molecular Structure Molar mass (g/mol)500.13414.07AppearanceColorless liquidDensity1.8 g/cm3Melting Point (° C.)40 to 50Boiling Point (° C.)133 at 6 Torr189 to 192Solubility in waterSoluble, 9.5 g/LSolubility in other solventsPolar organic solventsAcidity (pKa)<<00
Preliminary human health studies strongly indicate that these two perfluorinated compounds (PFCs) can bioaccumulate and pose significant risks. Both are water soluble, nonvolatile and persistent in the environment, causing them to be difficult to treat with conventional technology. For soil, excavation to acceptable landfills or incineration at high temperatures poses a high cost for remediation. For groundwater, extraction and adsorption on granulated activated carbon, a pump and treat procedure, would involve potentially tens of years to treat because a number of the PFCs are tightly bound to the soils. Other treatment alternatives are relatively experimental, expensive, and require groundwater extraction and ex-situ treatment (Hawley, Pancras, and Burdick, 2012).
Common chemical oxidation procedures have operational advantages for treatment in place but lack the reactivity and the oxidation potential to cleave the strong carbon/fluoride bond. Generally, activated persulfate (2.7 V), Fenton's reagent (2.8 V) and Perozone® (2.8 V) rank among the top chemical oxidation procedures for groundwater and soil in-situ treatment (ISCO), but do not reach the 2.9 V or above to cleave the Carbon-Fluorine bonds cleanly without leaving fragments. Ideally the process should be also capable of treating petroleum compounds spilled that required the use of foams composed of PFCs to fight aviation fires of actual crashes or for fire training exercises, without leaving behind high residual cations (e.g., iron) or anions (e.g., sulfates) which degrade the groundwater.
A number of recent laboratory studies attest to the effectiveness of chemical oxidation to destroy PFOS and PFOA, but the conditions are difficult to duplicate effectively in the field or secondary products were formed. The use of acoustic and UV light activation of persulfate would not be practical in situ but may be used ex situ. Hori et. al. (2005) found that advanced oxidation processes employing activated persulfate by heat efficiently degraded PFOA to fluoride ions and carbon dioxide, but did form minor amounts of shorter-chain perfluorocarboxylic acids, indicating complete mineralization may be possible with further oxidation. An aqueous solution containing 155 mg/L PFOA and 12 g/L persulfate was heated at 80° C. After 6 hours, aqueous phase PFOA concentrations were less than 0.6 mg/L (the detection limit). Fluoride ions and carbon dioxide were measured (molar ratios of 77.5% and 70.2%, respectively) to show complete mineralization. High sulfate residuals were also noted.
Ahmad et. al. 2012 found that hydroxyl radical (OH.), which is usually effective in oxidizing saturated and unsaturated carbon bonds in organic pollutants, is ineffective in degrading PFOA (kOH-PFOA<105 L mol−1 s−1). Several chelated hydrogen peroxides (CHPs) were effective in degrading PFOA: when (III)-catalyzed H2O2 decomposition used 1 M H2O2 and 0.5 mM iron (III), PFOA was degraded by 94% within 150 minutes; in reactions generating O2− or HO2− alone, PFOA was degraded rapidly. Hydroperoxide anion, the conjugate base of H2O2 (pKa 11.7) was generated by increasing the pH of a 2 M H2O2 solution to 12.7.